1. Field of the Invention
The present invention relates to an image processing apparatus capable of extracting a to-be-extracted target image portion included in an image targeted for processing, which is designed to prevent certain documents, such as banknotes, valuable securities, and confidential documents, from being illicitly copied.
2. Description of the Related Art
With recent improvements in performance of color copiers, there has been an increasing demand for a copier capable of preventing illicit copy of certain documents, such as banknotes and valuable securities, for counterfeit purposes.
One conventionally-known copy preventive technique employed in copiers is as follows. Whether or not a read image is subjected to image formation is determined by making a comparison between image data obtained by reading an image to be recorded on a recording paper sheet, such as an original, using an image reading apparatus, and image data on a specific image subjected to copy prevention that is stored in advance in a copier. When it is judged that image formation is not performed, an ongoing image formation is discontinued, or an image different from that having being read is formed.
As such a conventional art, Japanese Unexamined Utility Model Publication JP-U 5-85158 (1993) discloses an image processing apparatus capable of detecting a certain circular image such as a red seal printed in a banknote. This image processing apparatus functions as follows. Firstly, a red pixel included in a read image, as viewed in a main scanning direction, is detected, and the image composed of a plurality of red pixels is thinned out. Next, another red pixel located a predetermined distance away from said red pixel in the main scanning direction is detected. If it is assumed that the red pixel detected first and the red pixel detected subsequently are located at both ends of a line segment defining a diameter of a circle, the line segment is turned by a predetermined angle with respect to the central position of the circle in the main scanning direction. When the locations of the two pixels at both ends of the line segment fall in a predetermined range of pixel positions stored in a memory, the red pixels are judged to represent a circularly contoured image, namely, a red seal of a banknote.
As another conventional art, Japanese Unexamined Patent Publication JP-A 6-178098 (1994) discloses a similar image processing apparatus. This image processing apparatus functions as follows. Firstly, two pixels equivalent to both ends of a line segment defining a diameter of a circle in a main scanning direction are detected. Next, if it is detected that an image is present at an intersection of the circle and a line located a predetermined distance away from the center of the circle in a sub scanning direction, the assumption that the detected red image has a circular shape is judged to be correct. Moreover, in this image processing apparatus, the number of ON pixels falling in a predetermined space within the circle, namely, the number of red pixels, is counted, and it is judged whether or not the counted value is equivalent to the numerical value corresponding to specific image data stored in a memory. If equivalency is found, an image to be formed based on the detected pixels is judged to be a specific circular image, namely, a red seal of a banknote.
However, the above-described image processing apparatuses have the following disadvantages. In the former disclosed in JP-U 5-85158, a red seal of a banknote is identified by detecting a red circular image. This method is simple, but poses a problem that an image similar to a red circle of a banknote's red seal is inadvertently judged to be a banknote's red seal. This leads to poor judgment accuracy.
In the latter disclosed in JP-A 6-178098, the number of red pixels within a red circle image having being read is counted, and a comparison is made between the counted value and the numerical value corresponding to specific image data stored in a memory. This method provides excellent judgment accuracy, but requires a considerably large storage capacity of a memory for storing specific image data. This necessitates high-speed comparing means, resulting in a circuit configuration of the image processing apparatus being very complicated. Hence, the cost required for manufacturing the image processing apparatus is undesirably increased.